Once an oil or gas well has been formed it is common to install completion infrastructure in the well to control production of hydrocarbon fluids from a hydrocarbon-bearing formation surrounding the well to surface. The completion infrastructure may comprise a string of downhole tools joined by a string of production tubing to surface. The downhole tools are generally flow control or circulation devices such as packers, injection sleeves, production sleeves and the like. Such downhole tools are generally activated mechanically using a shifting tool attached to a work string to open, close or otherwise shift the position of sliding sleeves.
Typically a shifting tool is run on a work-string through the completion infrastructure to mechanically actuate the various downhole tools in a desired sequence. In order to mechanically actuate a downhole tool, the shifting tool is manipulated (via the work-string) from surface. Typically, actuation is achieved by locking the shifting tool onto profiles provided on the downhole tools and pulling (work-string in tension), pushing (work-string in compression), jarring, or rotating the work string to deliver the necessary force or impact to the downhole tool with which it is engaged.
As will be appreciated, it can be difficult to accurately control the operations of the shifting tool especially when it is situated at the end of several kilometers of work string and/or the shifting tool is located in a horizontal or highly deviated wellbore. In these situations it is usually not possible to accurately predict at surface whether the intended actuation has been successful. An additional disadvantage of these conventional shifting tools is the difficultly of use. For example, jarring down or slacking off to cause compression of the work-string risks that the work string will ‘catch’ on other downhole tools or land on an unintended component with some force thereby causing damage.
In view of the aforementioned problems with the use of conventional shifting tools, the Applicant developed a method for operating a downhole tool described in co-pending UK patent application no. 1205985.3, in which a work-string is first run into a well without actuating any downhole tools. The work-string is then used to operate a plurality of downhole tools in a desired sequence as it is pulled out of the well whilst being maintained in tension. This may provide an operator at surface with a more positive indication of the location of the shifting tool and a more accurate log of the operations performed using the shifting tool since every action requires a positive step in order to perform a subsequent operation. Such a system does not, however, provide conclusive confirmation that a shifting tool has actually actuated or shifted a sliding sleeve of a particular downhole tool. Moreover, such a system does not provide any information about the degree to which a sliding sleeve of a particular downhole tool has been actuated or shifted.
It is known to use Radio Frequency Identification (RFID) tags in oil and gas wells for conveying information between surface and a downhole tool. Although such RFID tags may have no built-in power supply or battery, such RFID tags do include active electronics (i.e. one or more electronic devices which are configured to electrically control a flow of electrons or an electrical current) for storing and communicating information to a tag reader. Such RFID tags generally include a memory, such as a memory defined on an integrated circuit, for storing information such a binary code which uniquely identifies the RFID tag.
For example, it is known to use “Type I” RFID arrangements and methods in which an RFID tag is located with or embedded into a downhole tool which is installed downhole. An RFID reader is subsequently run or conveyed downhole past the RFID tag. In response to reading information from the RFID tag, the RFID reader may communicate directly with the downhole tool causing the downhole tool to perform a downhole operation. Alternatively, the RFID reader may be incorporated into a shifting tool which is run or conveyed downhole and which is configured so that, in response to information read from the tag by the RFID reader, the shifting tool acts upon the downhole tool and causes the downhole tool to perform a downhole operation. Additionally or alternatively, the RFID reader may communicate the presence of the RFID tag to a surface controller. For example, the RFID reader may communicate the presence of the RFID tag to the surface controller over a cable such as a wireline or the like which supports the RFID reader, or the RFID reader may communicate the presence of the RFID tag to the surface controller along a work-string to which the RFID reader is connected. As such, the use of “Type I” RFID technology may permit bi-directional communications between a downhole tool and a surface controller. This may not only provide the capability to actuate a downhole tool, but may also provide real-time access to downhole measurement data, such as logging data.
“Type II” RFID arrangements and methods are also known in which an RFID reader is located with or embedded into a downhole tool which is installed downhole. An RFID tag is subsequently dropped or pumped downhole where the RFID reader reads the stored information from the RFID tag and, in response, actuates the downhole tool thereby causing the downhole tool to perform a downhole operation.
RFID tags may harvest energy from an electromagnetic field generated by an RFID reader and store the harvested energy in the form of charge on a capacitance located within the RFID tag. The charge is subsequently used to power the RFID tag for the wireless communication of the information stored in the memory of the RFID tag to the RFID reader. For example, the RFID tag may inductively couple the stored information to the RFID reader and/or may radiate the stored information to the RFID reader as an electromagnetic signal. In either case, the RFID tag wirelessly communicates the stored information to the RFID reader by modulating a harmonically varying electromagnetic field. For example, it is known for a RFID tag to modulate the amplitude or frequency of a harmonically varying electrical carrier signal according to a baseband information carrying signal, and to apply the modulated electrical carrier signal to an antenna of the RFID tag in order to wirelessly communicate information stored in the memory of the RFID tag to an RFID reader. Accordingly, such RFID downhole communication methods require the RFID reader and the RFID tag to have active electronics, for example integrated active electronics for the modulation of the electrical carrier signal. Active electronics may, however, be prone to failure in the harsh environment of an oil and gas well. For example, commercially available RFID tags and RFID tag readers and are generally only rated to 150° C. and may malfunction or may have a limited lifetime at temperatures in excess of 150° C. This is particularly true for a RFID tag or a RFID tag reader which is installed in an oil or gas well for the lifetime of the well which may extend for many years. Consequently, the use of RFID technology may be prohibited, or the reliability of RFID technology may be limited, at such temperatures.
Moreover, as temperature increases, the charge stored on the capacitance of a RFID tag may dissipate more rapidly thereby reducing a time period over which the RFID tag can wirelessly communicate information stored in a memory of the RFID tag to the RFID reader. In practice, this may impose a further restriction on the operating temperature range.
Accordingly, for the case of Type II RFID technology, it is not uncommon to drop or pump multiple RFID tags downhole to increase the probability that information stored on at least one of the RFID tags is wirelessly communicated to an RFID reader for actuation of a downhole device such as a downhole tool. Even then, actuation of the downhole device may not be sufficiently reliable depending on the operating temperature.
It is also known to use inductive coupling downhole for the wireless communication of data across a pressure barrier between different housing sections of a downhole tool string. For example, U.S. Pat. No. 6,021,095 entitled “Method and Apparatus for Remote Control of Wellbore End Devices” discloses the wireless communication of data along an axial direction across a pressure barrier between different housing sections of a downhole tool string by modulating a harmonically varying electromagnetic field coupled between a first coil mounted within a first housing section of a downhole tool string and a second coil mounted within a second housing section of a downhole tool string.
It is also known to use an array of electromagnets downhole to move objects along a throughbore along which the electromagnets are arranged. For example, US 2008/0053662 entitled “Electrically Operated Well Tools” discloses an operating member which includes an array of permanent magnets and which is moved along a throughbore using an array of electromagnets arranged along the throughbore. Similarly, US 2008/0202768 entitled “Device for Selective Movement of Well Tools and also a Method of Using Same” discloses a movable check valve which comprises a magnetizable material and which is moved along a throughbore using an array of electromagnets arranged along the throughbore.